Photodynamic therapy (PDT) is an advanced modality for the treatment of malignant tumors as it is widely used for clinical cancer treatments. This photodynamic therapy selectively destroys neoplastic lesions using cytotoxic reactive oxygen species (ROS) generated by light activation of the photosensitizer. One of the crucial factors determining the photodynamic therapy efficacy is the photochemical and photophysical properties of the photosensitizer.
The photosensitizers are classified into the following four main classes: porphyrin derivatives, chlorine, porphycenes, and phthalocyanines (Pcs). Among these, metallo-phthalocyanine (MPc) has attracted considerable interest, having a photodynamic (PD) property that can be readily tuned by the type of central metal ion and the functional groups introduced as a phthalocyanine (Pc) ring substituent. Zinc phthalocyanine (ZnPc) is known to exhibit a high photodynamic effect as it possesses a diamagnetic Zn(II) central metal ion whose d shell is fully occupied, by which the yield of triplet excited state with long lifetime essential for the generation of ROS becomes high. Moreover, ZnPc has a large absorption cross-section of light at the tissue-penetrating spectral range of 650-900 nm.
The biggest problem for most photosensitizers, including ZnPc, for the photodynamic therapy is the low physiological acceptance level due to their high hydrophobic characteristics responsible for the poor solubility in a bodily fluid. To overcome this problem, ZnPc derivatives, such as tetrasulfonated ZnPc (ZnPcS4), [1,2,3,4-tetrakis(α/β-D-galactopyranos-6-yl)-phthalocyaninato]zinc, tetra- and octa-triethyleneoxysulfonyl substituted ZnPc, have been designed to increase the water solubility. Moreover, various delivery vehicles including liposome, emulsion, and nanoparticles have also been developed to transport water-insoluble photosensitizers to targets.
However, these approaches require multiple and complex chemical functionalization steps, during which the photoactivity could be reduced by destroying the original electronic conjugation system of the photosensitizer. Another challenging issue is the realization of a photosensitizer that exhibits both photodynamic and photothermal effects simultaneously to conduct dual synergistic phototherapy, which is rarely found from a single photosensitizer.
Moreover, in the photodynamic therapy, a fluorescence imaging system is used to accurately determine the location of the photosensitizer in the body and the concentration of the photosensitizer accumulated in target cells, and thus it is possible to accurately treat cancer cells in a local area. Among various photosensitizers, zinc-phthalocyanine nanowires can absorb light at long wavelengths and generate much more reactive oxygen species due to the presence of zinc atoms, but the introduction into the fluorescence imaging system is not easy due to the absence of fluorescence.
Therefore, there is a need to develop a composite in which a material that exhibits fluorescence is introduced to overcome these drawbacks.